chalks

Chalks are soft, white, porous sedimentary rocks composed mainly of microcrystalline calcite (CaCO3). They form primarily from the accumulation of calcareous shells of coccolithophores and other microscopic algae, together with varying amounts of clay and silica. The rock is typically light-colored and easily crumbled.

Chalk formed largely in warm, shallow seas during the Late Cretaceous to early Cenozoic eras on continental shelves and margins. In some regions, thick chalk deposits are part of the Chalk Group, a widespread sequence that extends across parts of Europe, notably the United Kingdom and France.

Physically, chalk is soft (Mohs hardness around 3), fine-grained, and highly porous. It often contains nodules of flint and can weather to a white, powdery soil. Chalk beds commonly form prominent cliffs and serve as important aquifers, storing groundwater in its pore spaces and fractures.

Economically, chalk is used as a source of lime for cement and as agricultural lime to correct soil acidity. It has historic use as a building stone, particularly in southern England, and remains important as an aquifer in several regions. Chalk deposits also contribute to landscape and soil formation, influencing drainage and soil characteristics.

Notable exposures include the White Cliffs of Dover in England and chalk escarpments in the Paris Basin and Champagne regions of France, with widespread occurrences across Europe. The term chalk also appears in non-geological contexts to describe drawing material made from calcium carbonate, but in geology it refers to this soft, biogenic limestone.